An active matrix substrate for use in, for example, a liquid crystal display device is primarily constituted by electrode wires that are signal lines and scanning lines disposed in a matrix, pixel electrodes each provided for a pixel that is encircled by the signal lines and the scanning lines, and switching elements.
Each of the switching elements, if it is of a double terminal type, is connected to one of the pixel electrodes as well as to either one of the signal lines or one of the scanning lines, and if it is of a triple terminal type, is connected to one of the pixel electrodes, one of the signal lines, and one of the scanning lines. As the scanning line receives a predetermined voltage signal, the switching element is turned on, causing the image signal (electric potential) applied to the signal line to be transmitted to the pixel electrode. Well-known examples of switching elements for selectively driving pixel electrodes typically include TFT (Thin Film Transistor) elements of a triple terminal type and MIM (metal-insulating film-metal) elements of a double terminal type.
As shown in FIG. 9 through FIG. 11, in an active matrix substrate constituting a part of a liquid crystal display device that includes TFT elements (hereinafter, will be referred to simply as TFTs) as switching elements, the pixel is primarily constituted by electrode wires that are two signal lines 101 and two scanning lines 102 disposed in a matrix, a pixel electrode 103 provided for a pixel area that is encircled by the signal lines 101 and the scanning lines 102, and a TFT 104.
Note that FIG. 10 is a cross-sectional view taken along line F–F′ in FIG. 9 and that FIG. 11 is a cross-sectional view taken along line G–G′ in FIG. 9.
The TFT 104 has a gate electrode 106 connected to one of the scanning lines 102, a source electrode 107 connected to one of the signal lines 101, and a drain electrode 108 connected to the pixel electrode 103 and also to one of two terminals (transparent electrode layer 112) of a pixel capacitor (storage capacitor) 105a which will be discussed later. As a scanning signal is coupled to the scanning line 102, it drives the TFT 104, causing an image signal (video signal) coupled to the signal line 101 to be transmitted through a source electrode 107 and a drain electrode 108 and applied to the pixel electrode 103.
In the foregoing active matrix substrate, as shown in FIG. 11, the pixel capacitor 105a for storing the image signal applied to the pixel electrode 103 is constituted by a gate insulation film 110, as well as a pixel capacitor electrode (storage capacitor electrode) 105 and a transparent electrode layer 112 that are disposed opposing each other across a gate insulation film 110. The pixel capacitor electrode 105 doubles as a pixel capacitor common wire (storage capacitor common wire) that commonly connects a plurality of the pixel capacitors 105a together that are located parallel to the scanning lines 102, and is coupled to an opposite electrode on an opposite substrate (not shown) when incorporated in a liquid crystal cell.
FIG. 12(a) through FIG. 12(h) and FIG. 13(a) through FIG. 13(h) illustrate a manufacturing process of the active matrix substrate, where gate electrodes 106 and pixel capacitor electrodes 105 are formed on an insulating transparent substrate 109, and subsequently, a gate insulation film 110, a semiconductor layer 111, a n+-Si layer (corresponding to source electrodes 107 and drain electrodes 108), a transparent conductive layer 112, a metal layer 113, a protection film 114, an interlayer insulation film 115, and a transparent conductive layer constituting pixel electrodes 103 are deposited and patterned in this order. The transparent conductive layer 112 and the metal layer 113 connected to the source electrodes 107 of the TFTs 104 constitute signal lines 101.
In the active matrix substrate, the pixel electrode 103 is connected to the drain electrode 108 of the TFT 104 through a contact hole 116 formed through the interlayer insulation film 115. Meanwhile, the pixel electrode 103 (see FIG. 9) is separated from the signal lines 101 and the scanning lines 102 by the interlayer insulation film 115, permitting the pixel electrode 103 to overlap the signal lines 101 and the scanning lines 102 (see FIG. 9 and FIG. 10). It is known that this structure allows improvements on the aperture ratio and prevents insufficient alignment (disclination) from occurring in the liquid crystal, which would otherwise be caused by the shielding of the electric field generated by the signal lines 101 and the scanning lines 102.
Another typical, simpler method omits the step of forming the interlayer insulation film 115 and the pixel electrodes 103 which is provided on the film 115; the transparent conductive layer 112 is provided as pixel electrodes, and a large aperture for a pixel is formed in the protection film 114 which is deposited on the transparent conductive layer 112. This structure does not give as high an aperture ratio as the foregoing structure, but enables the active matrix substrate to be fabricated by a fewer steps and provides an advantage in terms of manufacturing costs.
The active matrix substrate prepared as above can find a wide range of applications which includes liquid crystal display devices. A specific example is a photosensor, serving as a photodiode, constituted by a semiconductor-layer-deposited element formed on the pixel electrode 103 so as to provide a PIN connection and a shot key connection; the pixel capacitor (storage capacitor) 105a of each pixel stores data in the form of electric potential as the diode increases its conductivity where it is irradiated with light while applying a predetermined d.c. voltage to the other terminal of the diode.
Another example is a sensor for sequentially reading electric charges that are generated by an conversion layer provided in place of the photodiode so as to directly convert light, x-ray, etc. to electric charges and then stored in the pixel capacitor 105a using a high voltage. An embodiment is disclosed, for example, in Japanese Laid-Open Patent Application No. 4-212458/1992 (Tokukaihei 4-212458; published on Aug. 4, 1992) where each pixel stores in its pixel capacitor 105a those electric charges generated by the conversion layer as data in the form of electric charges (as data in the form of electric potential) in accordance with the characteristics of an object. Similarly to a liquid crystal display device, by sequentially scanning the scanning lines 102, for example, the data stored in a pixel selected through the scanning lines 102 is read and transmitted through an active element (corresponding to the TFT 104) to a data line (corresponding to the signal line 101). At the other end of the data line, there is provided a circuit, such as an OP amplifier, for recovering a signal from the data; a set of image data is thus obtained from the object by the sensor.
The active matrix substrate, which is a precursor to the sensor in the foregoing example with no photodiode and no light-to-electricity conversion layer, can be manufactured at low costs without new investments in manufacturing tools and facilities, because the manufacturing process for liquid crystal display devices is applicable to sensors only by adjusting the dimensions of the pixel capacitor 105a and the time constants of the active element so as to obtain optimal results when used in a sensor.
For example, there is a demand for liquid crystal display devices used as computer display elements (monitors) to handle an increasingly large amount of information. To meet the demand, the display element (display section) is inevitably growing larger in size. Besides those applications as computer monitors, larger liquid crystal display devices are increasingly popular as monitors in AV (Audio Visual) and industrial systems. Meanwhile, display elements of medium to small sizes are increasingly required to produce highly clear images. In actual practice, these tasks are hard to tackle by means of designs.
Referring to FIG. 9 through FIG. 11, the following description will explain problems in solving the tasks more specifically. As the signal lines 101 and the scanning lines 102 are extended in response to the growing size of the display element, signal delays in the wires become no longer ignorable. Meanwhile, in medium to small sized display elements, the wires (signal lines 101 and scanning lines 102) inevitably come to have great resistance as the wires are scaled down in breadth to ensure a high aperture ratio with the already narrow pitches remaining unchanged, which results in more signal delays.
The problem of signal delays can be effectively solved by reducing the electrostatic capacity between wires, which is another factor to determine signal delays in the wires. However, the gate insulation film 110, which separates the signal lines 101 from the scanning lines 102, has further functions to decide the properties of the TFTs 104 and to constitute the pixel capacitors 105a; therefore, we cannot readily accept the use of a thinner gate insulation film 110 to reduce the electrostatic capacity per unit area.
The active matrix substrate for use in a sensor needs to clear more strict standards than the active matrix substrate for use in a liquid crystal display device; namely, noise, as well as signal delays, presents a problem that cannot be overlooked. Specifically, referring to FIG. 9 through FIG. 11, the pixel capacitor electrode 105, upon reading a signal from a target pixel, also receives a signal from an adjacent pixel that is connected commonly to the pixel capacitor electrode 105, which behaves as a noise superimposed on the target signal. Resolution is degraded by the noise, i.e., the signal obtained from the adjacent pixel, interfering with the target signal due to the electrostatic capacity between the pixel capacitor electrode 105 and the target pixel electrode 103. Alternatively, the electrostatic capacity between the pixel capacitor electrode 105 and the signal line 101 behaves as a noise in the signal line 101, and amplified by an amplifier for detecting the target signal, raising an obstruction in getting correct data. To obtain highly precise and accurate signals, the data is typically stored in greater amounts without causing an unnecessarily great increase in the pixel electric potential. This is effectively realized by setting the pixel capacitor to a great value; however, that setting would increase the impedance of a pixel capacitor common wire, resulting in aggravation of the aforementioned problems.
A more detailed explanation is given in the following about reasons that the impedance of a pixel capacitor common wire needs to be kept at a low value. In a case where the scanning line is disposed parallel to the pixel capacitor common wire, at an instant when the scanning line for a certain line is selected, all the pixels connected by means of electric capacity to the pixel capacitor common wire for that line behave as a load to the pixel capacitor common wire. In other words, in both cases of writing electric charges to a pixel and reading electric charges from a pixel, at an instant when the scanning line for a certain line is selected, the electric potentials of the pixels corresponding to the scanning line and the pixel capacitor common wire change all together, and therefore the electric potential of the pixel capacitor common wire constituting electrostatic capacitors with the pixels oscillates or shifts greatly off the value at which the electric potential of the pixel electrode common wire is desirably maintained. The electric potential of the pixel electrode common wire, if oscillates or shifts, can interfere with data, i.e., the electric potentials, for the pixels and thereby cause crosstalk.
Further, if the pixel electrode common wire crosses, and are connected by means of electric capacity to, numerous signal lines, the oscillating or shifting electric potential of the pixel electrode common wire negatively affects signals flowing through the signal line. The phenomenon is particularly manifest with liquid crystal display devices where the numerous signal lines are driven by high frequency alternating current.
For these reasons, in order to avoid negative effects on the electric potentials of the pixels and the signals flowing through the signal line and to stabilize the electric potential of the pixel capacitor common wire, the impedance of the pixel capacitor common wire needs to be kept at an extremely low value. This is achieved by, for example, composing the pixel capacitor common wire of a material of a small resistance.
Accordingly, a structure was conceived in which pixel capacitor common wires are disposed parallel to signal lines, whereas pixel capacitor common wires are disposed parallel to scanning lines in a typical structure. FIG. 14 and FIG. 15 illustrate, as an example, such a structure of an active matrix substrate for use in an x-ray sensor as disclosed in SID 98 DIGEST on pages 371 to 374. In the example, each pixel is surrounded by signal lines 201 and scanning lines 202 that are disposed in a matrix, and is provided with a pixel capacitor 205a in which a pixel electrode 203 opposes a pixel capacitor electrode 205 across a gate insulation film 210b. Further, pixel capacitor common wires 205b are disposed parallel to the signal lines 201.
In the foregoing structure, the signal lines 201 do not cross the pixel capacitor common wires 205b; therefore, the electrostatic capacity (load capacity) on the signal lines 201 can be decreased. The impedance of the pixel capacitor common wires 205b can also be decreased. As a result, the signal delays that occur in the signal lines 201 can be greatly diminished. In addition, crosstalk, which often raises a problem in a display device, can be prevented from happening. The structure, if adopted in an active matrix substrate for use in a sensor, can prevent the degradation in resolution caused by the noise generated by data from an adjacent pixel. Specifically, when a certain line is selected (a row of pixels parallel to the scanning line 202 to which a scanning signal is coupled so as to turn on the TFT 204 are selected) by means of a scanning line 202, the noise generated in the pixel capacitor common wire 205b may propagate along the signal line 201, but does not propagate along the scanning line 202, affecting no pixels connected to that scanning line 202. Therefore, the data obtained through the target pixel is free from negative effects from the other, simultaneously selected pixels.
However, to manufacture an active matrix substrate structured as above, additional steps should be included in the manufacturing process of an active matrix substrate shown in FIG. 12(a) through FIG. 12(h) and FIG. 13 (a) through FIG. 13 (h) before the formation of the pixel capacitor common wires 205b between the step of forming the scanning lines 202 (corresponding to those steps shown in FIG. 12(a) and FIG. 13(a)) and the step of forming the gate insulation film 210b (corresponding to those steps shown in FIG. 12(b) and FIG. 13(b)): namely, the additional steps are (a) the step of forming (depositing, patterning by photolithography, and etching) a transparent electrode film that is provided as the pixel capacitor electrodes 205 opposing the pixel electrodes 203 across the gate insulation film 210b, (b) the step of forming an underlying gate insulation film 210a prior to the formation of the pixel capacitor electrode 205, and (c) the step of depositing, patterning by photolithography and etching the gate insulation film 210b so as to form a contact section 205c between the pixel capacitor common wire 205b made of metal and the pixel capacitor electrode 205. Further, the gate insulation film 210b needs to be patterned for each pixel, separately from the other pixels; this requires a high level of precision in the patterning which can be achieved only through the use of costly photomasks and precise control of conditions in exposure to light and etching.
No protection film (corresponding to, for example, the protection film 114 in FIG. 10) is provided to protect the TFT 204; however, to improve the reliability of the device, such as an x-ray sensor, an inorganic protection film is preferably interposed between the TFT 204 and an interlayer insulation film 215 typically constituted by an organic film. In actual practice, an inorganic film composed of silicon nitride, for example, is interposed in the active matrix substrate for use in a conventional device. Consequently, the same number of steps are required after the completion of the formation of the gate insulation film 210b as in the conventional method shown in FIG. 9.
Therefore, the total cost will increase by the addition of the steps of depositing and patterning the transparent electrode film that will serve as the pixel capacitor electrodes 205, the addition of the steps of depositing and patterning (etching) the gate insulation film 210b, and the additionally required precision in the patterning of the gate insulation film 210b. Besides, if viewed in the balance with mass production, we cannot benefit a lot from the adoption of the above process and an resultant increase in the number of steps in the manufacture of active matrix substrates of small to medium sizes where design rules are relatively simple: the adoption would create another problem of reduced productivity of the manufacturing line because of the need for the manufacturing line to be adjusted so as to handle various processes depending on the sizes of active matrix substrates.
Besides, if the pixel capacitor electrodes 105 are provided below the gate insulation film 110 as shown in the arrangement shown in FIG. 10 and FIG. 11, a simple method of providing a transparent conductive layer 112 as pixel electrodes become applicable as previously mentioned, except the step of forming the interlayer insulation film 115 and the pixel electrodes 103 thereon. By contrast, if the pixel capacitor common wires 205b are provided on the gate insulation film 210b as shown in the arrangement shown in FIG. 14 and FIG. 15, such an simple method cannot be applied.
Further, in the active matrix substrate shown in FIG. 14 and FIG. 15, a through hole which is large enough to form a supplementary capacitor is provided in the interlayer insulation (polymer) film 215 that is deposited in a thickness of 2 μm or more; if such an active matrix substrate is used in a liquid crystal display device, since the through hole is located in an important part for image display (the part where light passes in the case of a liquid crystal display device of a transparent type), the through hole disturbs alignment of the liquid crystal. Therefore, contrast degradation and other serious problems in terms of display quality are highly likely to occur.
Meanwhile, U.S. Pat. No. 5,182,620 (corresponding to Japanese Laid-Open Patent Application No. 3-288824/1991 (Tokukaihei 3-288824), published on Dec. 19, 1991) discloses an active matrix substrate including TFTs of a top gate structure (normal stagger structure) as switching elements, wherein pixel electrodes are disposed on an interlayer insulation film to achieve a high aperture ratio, and supplementary capacitor wires are disposed parallel to signal lines. Besides, the semiconductor layer of TFTs and the lower electrodes constituting capacitors are fabricated from a polycrystalline silicon thin film through patterning and other steps. Gate bus wires, gate electrodes, and upper electrodes constituting capacitors are also fabricated from a polycrystalline silicon thin film through patterning and other steps. Each of the supplementary capacitor is formed by providing a lower electrode constituting the capacitor so as to oppose an upper electrode constituting the capacitor across an insulation film.
However, the arrangement cannot be applied to an active matrix substrate with TFTs of an amorphous silicon type for the following reasons. If at least either of the lower and upper electrodes constituting a capacitor are formed from amorphous silicon, stable capacity properties are not available due to the replacement of the polycrystalline silicon thin film for an amorphous silicon thin film. More specifically, amorphous silicon has a lower conductance than polycrystalline silicon, and the capacity is more likely to be changed by voltage in a TFT of an amorphous silicon type.
Besides, an active matrix substrate including TFTs of an amorphous silicon type better restrains leak currents from TFTs caused by light projection onto the active matrix substrate, if the TFTs are of an inverted stagger structure, instead of a normal stagger structure.